The present invention relates to improvement of a power steering apparatus for assisting steerage by using actuators such as hydraulic cylinder, hydraulic motor, and the like, as a generation source of the hydraulic power for the hydraulic pump to be driven by rotation of an electric motor.
FIG. 1 is a block diagram showing an essential constitution of a conventional power steering apparatus for assisting steerage by an actuator using a hydraulic pump rotationally driven by an electric motor as a generation source of the actuating oil pressure. In this power steering apparatus an electric motor 4 is driven by a voltage applied by a motor driving circuit 2 according to a target value of the applied voltage indicated by a target determining unit 1c. The electric motor 4 rotationally drives a hydraulic pump 5 to generate the actuating oil pressure. The control valve 6 controls the pressure of the actuating oil to be sent under pressure to the pipelines 6a and 6b which are led to a hydraulic cylinder.
When the steering wheel 7 is manipulated to operate the gear apparatus including the pinion gear 6c provided at the lower end of the steering shaft, the control valve 6 controls the pressure of the actuating oil to be sent under pressure to the pipelines 6a, 6b led to the hydraulic cylinder. By this step, the hydraulic cylinder is operated to generate the steering assistant force proportionate to the operating amount in the direction of the operation of the steering wheel 7.
The actuating oil circulates in the hydraulic circuit constituted by the hydraulic pump 5, control valve 6, hydraulic cylinder and oil supply tank 5a, etc.
To the motor driving circuit 2 there is connected a motor current detecting circuit 3 for detecting the current led to the electric motor 4, and the motor current detecting signal is inputted to the no-load current detecting unit 1a and the operator 1b. The no-load current detecting unit 1a detects to hold the motor current detection signal in no-load time in consideration of the motor current which varies depending on the oil temperature. The operator 1b deducts the signal from the motor current detection circuit 3 by the signal from the no-load current detecting unit 1a and inputs the variation amount of the motor current detection signal to the target determining unit 1c. The target determining unit 1c is to control, by the amount of variation of the motor current detection signal, the voltage to be applied to the electric motor 4 by switching to high or low level according to the amount of variation of the motor current detection signal.
According to the conventional power steering apparatus having the constitution as described above, when the steering wheel 7 is not operated and no steering assistant force is required (the current running to the motor 4 is small), the target determining unit 1c reduces the voltage to be applied to the electric motor 4 by one step to decrease the output of the electric motor 4, as shown in FIG. 2. And, when the steering wheel 7 is manipulated to operate the control valve 6, the actuating oil pressure rises to increase the load and the current running in the electric motor 4. When the variation amount of the motor current detecting signal outputted by the operator 1b reaches the predetermined amount, the target determining unit 1c increases the voltage to be applied to the electric motor 4 by one step to increase the output of the electric motor 4, thereby increasing the steering assistant force.
However, in the conventional power steering apparatus, when, as described above, the steering wheel 7 is manipulated to close the control valve 6 as shown in FIG. 3A and the actuating oil pressure starts to rise, the hose lying between the hydraulic pump 5 and the control valve 6 starts to expand its volume due to its elasticity. Consequently, although the discharging amount of the hydraulic pump 5 does not show change (FIG. 3B), the exit flow amount of the control valve 6 (exit flow amount of the actuator) decreases as shown in FIG. 3C-A, during the expansion of the inner volume of the hose (e.g., about 0.2 second), and the rise of the actuating oil pressure on the high pressure side of the control valve 6 (actuating oil pressure on the entrance side) becomes slow as shown in FIG. 3 D-B.
For example, assuming the hose swelling amount to be 10 cm.sup.3 /m, the pump delivery amount to be 2000 cm.sup.3 /min., and the hose length to be 50 cm when the actuating oil pressure increases from 0 to 6 MPa, EQU (10 cm.sup.3 /m).times.0.5 m.div.(2000 cm.sup.2 /60 seconds)=0.15 second.
This means that the time of 0.15 second is required before the actuating oil pressure rises to 6 MPa.
When the rising of the actuating oil pressure on the high pressure side of the control valve 6 becomes slow, the rising of the load of the electric motor 4 is also dulled through the hydraulic pump 5, thereby retarding the increase of the current led to the electric motor 4. Because of this, time is required from the manipulation of the steering wheel 7 to the increase by one step of the voltage to be applied to the electric motor 4 and the increase in the steering assisting force, so that there is a problem that the steering wheel catches in abrupt manipulation. This effect strongly appears especially when the delivery flow amount of the hydraulic pump 5 is set to be small, in a case where the output of the electric motor 4 is reduced so as to suppress energy consumption.